widget - 尝试理解 flutter 演示代码中的小部件对象

Question

我只是想学习 flutter，flutter 演示代码中有一段代码，是您在创建新的 flutter 项目时得到的，我不明白:

标题:新文本(widget.title)

我不明白小部件来自哪里，因为它没有被声明、定义或初始化。它指的是这个文本:

home: new MyHomePage(title: 'Flutter 演示主页')

但是为什么，这与上下文有关吗？如果它是预定义的，我可以如何以及在哪里使用它。由于 flutter 中的所有内容都是 Widget，因此很难向 Google 询问这个问题。

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
   });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              'You have pushed the button this many times:',
            ),
            new Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
   );
  }
}    

Answer 1

MyHomePage 具有最终字段 title，您可以在构造函数中声明该字段 - MyHomePage(title: 'Flutter Demo Home Page')``_MyHomePageState - 这是有状态小部件的状态。在每种状态下，您都可以使用 widget 来获取 StatefulWidget